1. Field of the Invention
This invention relates to a process for the preparation of sulphur dioxide by the thermal decomposition of metal sulphates in fluidized bed reactors with reducing agents containing sulphur and energy suppliers.
2. Background Information
Fluidized bed reactors have on the whole been found to be the most satisfactory apparatus for the thermal decomposition of metal sulphates, in particular of sulphuric acid-containing salt mixtures rich in iron sulphate monohydrate of the kind obtained in the recovery of sulphuric acid from dilute titanic acid or mordanting or pickling liquors. They are distinguished by their high specific output and relatively low maintenance costs.
Both sulphur and iron pyrites, as well as carboncontaining fuels, such as coal, coke and fuel oil, are used as reducing agents and energy suppliers for the highly endothermic reactions. It is advantageous to use sufficient reducing agent containing sulphur to produce a gas mixture which, when mixed with air, contains at least 6 vol.-% of SO.sub.2 and 6 vol.-% of O.sub.2 and is suitable for the formation of SO.sub.3 in contact with the sulphuric acid catalyst. The amount of carbon-containing fuels put into the process is determined, not only by economical considerations, but also by the fact that decomposition of the metal sulphate must not yield a larger amount of SO.sub.2 than can be used as such or as sulphuric acid. It is therefore frequently necessary to use sulphur-containing reducing agents and carbon-containing fuels at the same time.
To ensure as far as possible complete decomposition of the metal sulphates to metal oxides, SO.sub.2 and O.sub.2 it is necessary to provide for a sufficiently long dwell time in the reaction zone and a sufficient supply of energy. The latter requires thorough mixing of the metal sulphates with the reducing agents and fuels, while the former requires the reaction mixture to be introduced directly into the fluidized bed which has a high solids content. One method which has proved to be reliable for this purpose is the injection of a metal sulphate suspension into the fluidized bed layer from the side. This method has the advantage that a suspension is more easily handled than solid substances, especially if the metal sulphates contain sulphuric acid as residual moisture. Additional problems arise if very fine flotation iron pyrites is used instead of the nowadays almost unobtainable fine iron pyrites which is relatively coarse by comparison. In DE-A No. 3,328,710, it is proposed to deal with this problem by moistening the flotation iron pyrites with sulphuric acid and injecting it into the fluidized bed layer.
If a fluidized bed reactor is very large, problems arise because uniform mixing of the energy consuming starting materials and the energy supplying starting materials in the fluidized bed can at best only be achieved approximately. With a view to obtaining uniform distribution of the metal sulphates and of the flotation iron pyrites and at the same time enabling trouble-free introduction of the flotation iron pyrites into the fluidized bed reactors to be achieved, it is proposed in DE-A No. 3,328,710 to prepare granules in particular of metal sulphates containing sulphuric acid and to cover these with flotation iron pyrites. These granules composed of a hygroscopic core and a protecting shell of iron pyrites can readily be introduced into the reactors, but problems arise if a relaitvely large quantity of carbon-containing fuels are required for providing the necessary amount of energy for the reaction. If the fuels are introduced in large quantities into the fluidized bed layer from the side or from below. Then localized zones of highly reducing conditions are created which considerably interfere with the operation of the reactor, whereas in other zones the fluidized bed cools down due to lack of fuel so that insufficient sulphate decomposition takes place.